The rapid expansion of the number of cellular radiotelephones coupled with the desire to provide additional services has prompted the use of an improved transmission technique, time division multiple access (TDMA). TDMA increases system capacity over the current analog system through the use of digital modulation and speech coding techniques. A TDMA transmission is comprised of many time slots.
A linear modulation technique, .pi./4 differential quadrature phase shift keying (.pi./4 DQPSK), is used to transmit the digital information over the channel. The use of linear modulation in the U.S. Digital Cellular system provides spectral efficiency allowing the use of 48.6 kbps channel data rates. .pi./4 DQPSK transmits the data information by encoding consecutive pairs of bits, commonly known as symbols, into one of four phase angles (.+-..pi./4, .+-.3.pi./4) based upon gray encoding. These angles are then differentially encoded to produce an 8 point constellation.
Transmitters designed for use in the U.S. Digital Cellular system are required to operate in both the analog and digital modes. The digital mode uses the .pi./4 shift DQPSK modulation, and can be implemented using a linear transmitter. The analog mode uses conventional frequency modulation and allows the use of higher efficiency non-linear transmitters.
Conventional linear amplifiers are inherently less efficient than their constant envelope counterparts due to the types of signals they must amplify. A constant envelope amplifier is required to put out a signal at only one power level over time. It can therefore be optimized for peak efficiency at that power level. This optimization entails placing a load impedance on the device such that, at the designed power out, the AC collector voltage magnitude is close to or even exceeds the DC supply voltage. In this condition, the amplifier is close to or actually in saturation and has optimum efficiency.
The linear amplifier must amplify signals at power levels that vary over time, with whatever amplitude modulation that has been impressed upon the input signal. No saturation is allowed in the linear amplifier, or there will be severe distortion of the envelope. This distortion causes loss of amplitude information and spreading of the transmit spectrum into adjacent channels. The amplifier circuit must operate such that at peak power out, the amplifier is not in saturation. While it is possible to optimize for good efficiency at peak power out, the efficiency falls off rapidly as power out falls.
This creates a problem for the U.S. Digital Cellular radio which is intended to operate in both linear and constant envelope modes. When compared to current analog radios with constant envelope amplifiers, efficiency will be much lower in the digital radio. In the linear mode, efficiency is optimized for peak power out, but the signal spends only a short time there. Average efficiency will be lower than peak power efficiency because of this. For digital cellular this is not a severe limitation since a TDMA system is used and the transmitter is only on 1/3 of the time (only every third time slot is used by the radio). Even if the average efficiency is poor in this mode, transmit current is not significantly worse (and may be better) than a conventional analog radio.
The problem arises when this same radio is used for an analog call with a constant envelope signal. Now the transmitter is on for 100% of the conversation time and, since it operates at an average power 3.2 dB below the optimum power out, efficiency is poor. Data on test circuits shows drops in the 12 percentage point range. This translates into a substantial increase in transmit current. The increased current requirements will substantially decrease the time that a battery powered radiotelephone will be useful. There is a resulting need for an amplifier circuit that operates efficiently in both linear and constant envelope modes.